Inertial separating inlets for gas turbine engines for preventing the ingestion of supercooled water, ice, snow and other debris into an aircraft engine are well known. In such installations, a duct is typically located adjacent the nacelle containing the engine and extends generally longitudinally of the engine. The duct includes an air admitting opening at the upstream end and a bypass discharge opening at the downstream end.
Air enters the engine nacelle via an opening in the duct wall with the engine air forced to make a relatively sharp turn prior to exiting the duct. The portion of the admitted air passing downstream and exiting the bypass opening carries the liquid, ice or other debris which has been separated from the engine air by inertial forces.
As is well known to those skilled in the art, an increased ratio of bypassed air to engine air results in an increased overall momentum drag for the engine nacelle-duct structure. It is therefore desirable to reduce the bypass ratio of the separating duct especially during periods of high speed aircraft operation.
One important consideration with regard to the modification of the bypass flow ratio of the separating duct is the potential for ice formation within the duct on any flow deflecting or other surfaces present therein. Such interior ice formation may restrict or alter the function of the separating duct, and thereby adversely affect engine performance or reliability.
It is common in fixed wing aircraft applications to use the air pressure rise resulting from aircraft speed to drive the flow through the separating duct for protecting the engine. Such an arrangement is ineffective in rotating wing aircraft which frequently require protection during hover, climbout, etc. Such rotating wing aircraft require a flow inducer or other means in the duct to ensure a continuing flow of bypass air in order to achieve debris separation during hover as well as to prevent flow reversal in the bypass discharge opening during low speed flight.
The requirement for a flow inducer for hover and low speed operation in rotating wing applications leads to an increased bypass flow ratio at relatively high rotor craft speeds and hence an increased aerodynamic drag during such operation.